1. Field of the invention
The present invention relates to a composite filter and a duplexer, in which a plurality of filters is formed in a single dielectric block, and a communication apparatus using the same, used in the microwave band of a mobile phone or the like.
2. Description of the Related Art
Conventionally, a duplexer, in which resonators forming a transmitting-side filter and resonators forming a receiving-side filter are integrally molded in a single dielectric block formed of dielectric materials with an equivalent dielectric constant, is known. Such a duplexer is formed by disposing a plurality of resonator holes having inner conductors on the inner peripheral surfaces thereof between opposing end surfaces of the dielectric block having an outer conductor on an external surface thereof, in which each of the inner conductors is formed for serving as a one-end open resonator which is separated from the outer conductor on one end, whereas it is short-circuited with the outer conductor on the other end.
Since the transmitting-side filter and the receiving-side filter have different frequencies in such a conventional duplexer, the lengths of resonators forming the respective filters are naturally different, in which there is a difference in the length of the resonator length direction between the portion forming the transmitting-side filter and the portion forming the receiving-side filter, whereby the unit has a configuration with a step portion on the outline of the dielectric block. For example, when a central frequency of the transmitting-side filter is 836.5 MHz, a central frequency of the receiving-side filter is 881.5 MHz, if a dielectric material whose relative dielectric constant ∈r is 92 is used, the resonator length of the transmitting-side filter is 9.34 mm, and the resonator length of the receiving-side filter is 8.86 mm, resulting in generating a step, which is 0.48 mm, on the outline of the dielectric block.
Furthermore, conventionally, there is provided a duplexer, in which a portion of an inner conductor is eliminated to form an open end of a resonator inside a dielectric block, and adjustment of a location for forming the open end allows a transmitting-side filter and a receiving-side filter to be formed in a single dielectric block formed of materials having the equivalent dielectric constant, with no steps on its outline. (Japanese Unexamined Patent Publication No. 8-330806).
However, regarding the former type of the above described prior art duplexer, in which the end of the dielectric block is an open end, when the dielectric block is molded, various kinds of molding metal dies with high precision, having step portions according to the frequency difference between the two filters, are needed, so that die cost and molding cost increase. Additionally, due to its complicated configuration having a step on the outline, looseness and deviation are likely to occur in processing, assembly, measurement, mounting, or the like after molding, so that work such as location determination is complicated so as to increase production cost.
Meanwhile, as the later type, in the prior art duplexer, in which the open end of a resonator is formed inside the dielectric block, when the difference between the frequency of the transmitting-side filter and that of the receiving-side filter is large, the open end (that is, a portion where no inner conductor is formed) of the resonator forming the filter of a lower frequency is needed to be formed in a location which is very deeply recessed from the open-side end surface of the dielectric block, leading to difficulty in forming the non-inner-conductor portion or forming with high precision in a specified location.
To overcome the above described problems, preferred embodiments of the present invention provide a composite filter, a duplexer, and a communication apparatus, which are low-cost, small, and have satisfactory characteristics.
One preferred embodiment of the present invention provides a composite filter comprising: a single unitary dielectric block having an outer conductor on the external surfaces thereof; a plurality of inner conductors disposed in the dielectric block; a plurality of filters with different frequencies respectively comprising resonators made of at least one of said inner conductor; a first portion of said dielectric block constituting at least one of said filter comprising a first dielectric material; a second portion of said dielectric block constituting the other one of said filter comprising a second dielectric material; and the dielectric constant of said first dielectric material and said second dielectric material being different from each other.
The above described composite filter may be a duplexer in which at least one of said filter comprising said first portion of said dielectric block and said first dielectric material is a transmitting-side filter; and at least one of said filter comprising said second portion of said dielectric block and said second dielectric material is a receiving-side filter.
Another preferred embodiment of the present invention provides a communication apparatus comprising: a transmission circuit; a reception circuit; an antenna; the above described composite filter or duplexer: said at least one of said filter comprising said first portion of said dielectric block and said first dielectric material having a first input and a first output, said first input being connected to said transmission circuit, and said first output being connected to said antenna; and the other one of said filter comprising said second portion of said dielectric block and said second dielectric material having a second input and a second output, said second input being connected to said antenna, and said second output being connected to said reception circuit.
In the composite filter or the duplexer having the above described structure, a plurality of filters with different frequencies are formed in the single unitary dielectric block, in which the portion forming at least one of the filters is formed of a dielectric material having a dielectric constant different from that of the portion forming the other filter. In this structure, appropriate selection of a dielectric constant of the portion forming each filter corresponding to the frequency of each filter allows the physical resonator length of each resonator having a different frequency to be made into the equivalent length, even when the open end of each resonator is formed in the same position. More specifically, the present invention is achieved by using dielectric materials, in which a dielectric constant of the portion forming a lower-frequency filter is larger than that of the portion forming a high-frequency filter. That is, the single dielectric block including a plurality of filters with different frequencies can be made into rectangular parallelepiped configuration having no steps.
In addition, when the inner conductor is eliminated to form an open end, the open end of each resonator can be formed substantially in the same position near the open-side end surface, so that formation of the non-inner-conductor portion can be easily conducted with high precision.
Furthermore, combining the method of using dielectric materials having different dielectric constants in the present invention with other designing methods such as changing the position of the open end can enhance freedom in design, so that various kinds of characteristics can be easily obtained by using a single dielectric block without any step formed.
In the composite filter and the duplexer according to the present invention, a plurality of filters are formed in the single unitary dielectric block integrally molded and burned, in which the number of components are less than that in a unit with each filter formed by an individual dielectric block so as to permit easy handling, or to make it unnecessary to bond a plurality of the filters together by soldering or the like, leading to cost reduction.
Furthermore, since the communication apparatus according to the present invention includes the composite filter or the duplexer having the above-described characteristics, low cost, miniaturization, and satisfactory characteristics can be achieved.
Other features and advantages of the present invention will become apparent from the following description of the invention which refers to the accompanying drawings.